One of the most brilliant books I have ever read in my life is Gary Drescher's book Good and Real: Demystifying Paradoxes from Physics to Ethics. I highly recommend it: it covers naturalistic perspectives on consciousness, ethics, and manages to be fairly brief on each topic even though it explicates numerous philosophical problems and blossoms with fresh new solutions to many of them. I say that to say this: Reading pages 323-327 of that book is going to be a pre-requisite to reading this blog post, as what I attempt to do here is to explicate Gary's argument for Modal Realism. If you don't have the book, all of those pages except one are available here. Anyway, what I'm writing here is an attempt to put his argument into a series of syllogisms. What I say here is my own interpretation of his work and I could be mistaken. Also, I have speculated and filled in some gaps about what he meant when I felt it was necessary.
I take Drescher's argument to boil down to one simple syllogism, though in order to be valid this syllogism requires lots of justification through further syllogisms. Here's the basic argument:
1. Two things are the same when no difference could ever [even in principle] be detected between them.
2. No difference can be detected (even in principle) between the objective ontological status of logically possible worlds.
Conclusion: Therefore, the objective ontological status of this world and all possible worlds is the same.
A common sense way to phrase the conclusion, I think, is that all possible worlds are just as real as this one.
Now, we've got some serious philosophical legwork to do. Since the conclusion follows from the premises, we only have to worry about whether the premises are true. I take premise one to be uncontroversial. But what about premise 2? Is it true? Here's Drescher's argument for it:
1. Any statement which is not true by definition or theoretically verifiable/falsifiable is meaningless.
2. The statement that "The universe exists" in the sense that it has some external "spark of existence" as Drescher puts it, is not true by definition nor is it even theoretically verifiable/falsifiable.
Conclusion: Therefore, the statement "The universe exists" (as defined above) is meaningless.
Now, the first premise expresses a version of something known as the Verification Principle. This principle is highly controversial in philosophy, as the link I gave will show you. However, Michael Martin has valiantly defended this principle in his book Atheism: A Philosophical Justification and personally I suspect that some form of the verification principle is correct.
What about the second premise? What justifies the contention that there we could never know of the universe having an external "spark of existence" as Drescher says? Drescher runs a thought experiment similar to this one that I have written about before:
Imagine that you "invent" your own universe. You write down the physical laws of your imaginary universe, and you work out the equations to figure out how it would evolve over time. Eventually your equations show that your universe develops planets and life at some point in its history. Further equations prove that intelligent, humanoid creatures would evolve in this universe. Working even more equations reveals that a pair of these humanoids are having a conversation about their universe, wondering why it exists. They are not troubled, or even aware, that their universe "doesn't exist." In their eyes, their universe does exist. It is quite real to them. Their "imaginary" universe seems just as real to them as our "real" universe seems to us. We have no way to tell what "exists" except our experiences. And yet experiences exist within universes that we would call "imaginary": For as our thought experiement shows, 'imaginary' beings in 'imaginary' universes still experience their universe as real. And therefore they have the same evidence that their universe exists as we have that our universe exists.
Something must be explained here: Am I concluding that we can't know whether the universe exists? No, obviously the universe does exist. But maybe our intuitions about what existence means have lead us astray. So, what is existence? Drescher proposes that we call things "real" only when they can have an effect of some sort upon us. Defined this way, of course our universe is real, since the universe is just a label for everything we call real (in the sense of having, or being able to have, an effect upon us). Notice that when we use this definition of "real" an imaginary being (think three-eyed monster alien) in a possible world (with its own unique set of galaxies and planets) could also rightly refer to itself and its surroundings as being real. We, on the other hand, refer to it as not real because such a thing has no effect upon us. Even as we contemplate such alien worlds, it is not the worlds themselves which affect us, but only our mental representations of them that were generated by our brains.
Some have objected that the theory of Modal Realism violates Occam's razor. I think not. First of all, the principle that "the simplest explanation is most probably correct" does not apply if one proves logically that there is only one explanation, as I believe I have just done. Nor is it even clear that Occam's razor would be violated even if there were other possible explanations of existence. I mean, if someone were proposing a theory that granted a "spark of existence" to every possible world, such a theory would run afoul of Occam's razor. But modal realism actually denies that any universe, even ours, has anything extra that other universes do not. It is, then, simpler than alternative theories. Our universe, ontologically, is no different than any other abstraction.